Arrangement for synchronization of a stream of synchronous traffic delivered by an asynchronous medium

ABSTRACT

An ATM cell constructor ( 100 ) of an ATM transmitter assembles a stream of frames of constant bit-rate traffic received on a listen TDM bus ( 102 ) into cell payloads ( 1104 ) using ATM adaptation layer  1  (AAL 1 ). Once every eight cells, the AAL 1  structured data transfer (SDT) cell constructor layer ( 112 ) introduces a one-octet SDT offset pointer ( 1120 ) into the payload. This pointer designates traffic-block (TDM frame) boundaries. The payload with an attached ATM header forms an ATM cell, and the constructor transmits a stream of the ATM cells to an ATM cell deconstructor ( 2100 ) of an ATM receiver. The deconstructor disassembles the payloads of the received ATM cells and transmits the stream of frames of constant bit-rate traffic on a talk TDM bus ( 102 ). In response to each received SDT offset pointer, the deconstructor&#39;s time slot interchanger (TSI  2108 ) resets to the start of frame-processing, thereby aligning frames formed by the TSI with the received frames. The deconstructor ( 2100:2104 ) detects occurrence of TDM frames on the talk TDM bus and synchronizes (aligns) transmission of the frames formed by the TSI with the TDM frames on the talk TDM bus. The listen and talk TDM buses are thereby synchronized with each other.

TECHNICAL FIELD

This invention relates generally to packet-switching systems, such asasynchronous transfer mode (ATM) systems, and specifically totransmission-delay variations in such systems.

BACKGROUND OF THE INVENTION

Today's business communications environment consists of two separatenetwork infrastructures: a voice network (such as a private branchexchange (PBX)) characterized by real-time, high-reliability, constantbit-rate (CBR) connections; and a data network (such as a packetnetwork) characterized by high-bandwidth variable bit-rate (VBR)connections. Business needs for simplified maintenance, management, andaccess to information on diverse networks are forcing the convergence ofthese networks along with a new class of real-time multimedia networks.Asynchronous transfer mode (ATM) provides a single infrastructure thatcost-effectively and flexibly handles both switching and transmissionfor the traffic types mentioned above (voice, video, and data) for bothlocal-area networks and wide-area networks. The evolving networkconvergence requires the adaptation of the legacy PBX voice traffic toATM. Voice telephony over ATM (VTOA) specifications allow adaptation ofcompressed or uncompressed voice pulse-code modulated (PCM) data streamsinto streams (virtual circuits) of CBR cells.

An ATM cell, regardless of the traffic it carries, is a packet 53 octetslong: 48 octets of payload attached to a 5-octet header. The headercontains addressing and management information used to direct the cellfrom source to destination and to ensure that the negotiated aspects ofthe traffic-flow through the ATM network are met. CBR traffic isassembled into cell payloads using ATM Adaptation Layer 1 (AAL1). TheAAL1 cell constructor layer uses the first octet of the payload for itsheader and the remaining 47 octets to carry CBR information. Once everyeight cells, the AAL1 structured data transfer (SDT) cell constructorlayer introduces a one-octet pointer into the CBR payload. This pointerdesignates traffic-block boundaries. It is used at the receiving end togenerate framing signals for devices that convert the ATM traffic intoT1 or E1 (telephony trunk) traffic. ATM cell construction is thencompleted by attaching the ATM header to the payload.

Known existing devices that convert the ATM traffic into T1 or E1traffic do not synchronize (align) the received blocks of traffic withthe appropriate bits on a T1, E1, or TDM bus that is synchronized toanother source (e.g., the destination synchronization source).

SUMMARY OF THE INVENTION

This invention is directed to solving these and other problems anddisadvantages of the prior art. According to the invention, the receivedblocks of traffic are synchronized with the destination synchronizationsource. The synchronization is advantageously effected using the SDTblock-boundary pointer. Generally according to one aspect of theinvention, synchronous information received by a receiver over anasynchronous communications link is synchronized with a synchronouscommunications medium at the receiver as follows. The receiverasynchronously receives a stream of information, including thesynchronous information, and an indication (e.g., the SDT block-boundarypointer) of where in the stream of information occurred a boundarybetween blocks (e.g., time-division multiplex (TDM) frames) of thestream of information, and transmits the received stream on the medium(e.g., a TDM bus). The receiver also detects where boundaries betweenblocks (e.g., between TDM frames) for information occur on the medium,and uses the indication (the pointer) to synchronize transmissions fromthe receiver on the medium of information at the boundary in the streamof information with occurrence of a boundary between blocks on themedium. Illustratively, both the indication and the boundaries betweenblocks for information on the medium represent boundaries between TDMframes of received traffic, so that the receiver aligns TDM frames ofreceived traffic with TDM frames on the medium. If the receiver includesa time-slot interchanger (TSI) for reordering the time slots of TDMframes of received traffic, the TSI is reset by each indication, so thatTDM frames generated by the TSI for transmission on the medium alignwith the TDM frames of received traffic.

According to another aspect of the invention, a first synchronouscommunications medium (e.g., a TDM bus) at a receiver of an asynchronouscommunications link (e.g., an ATM link) is synchronized with a secondsynchronous communications medium (e.g., a TDM bus) at a transmitter ofthe asynchronous communications link as follows. The transmitter detectsa boundary (e.g., a framing signal) between blocks (e.g., TDM frames) ofinformation in a stream of information that it is receiving from thesecond medium, and transmits the information asynchronously from thetransmitter to the receiver with a first indication (e.g., an SDTpointer) of where in the stream of information the boundary occurred.The information and the first indication are received by the receiver,which transmits the information on the first medium. The receiver alsodetects where boundaries (e.g., framing signals) between blocks (e.g.,TDM frames) for information occur on the first medium, and in responseto receipt of the first indication the receiver synchronizestransmission on the first medium of the information at the boundaryindicated by the first indication with occurrence of a boundary betweenblocks on the first medium. Again, if the receiver includes a TSI, theTSI is preferably reset to the beginning of frame generation by eachreceived first indication.

The invention includes both a method of as well as a correspondingapparatus and a computer readable medium that contains software which,when executed in a computer, causes the computer to perform the method.The apparatus preferably includes an effector—any entity that effectsthe corresponding step, unlike a means—for each method step.

These and other features and advantages of the present invention willbecome more apparent from the following description of an illustrativeembodiment of the invention considered together with the drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of an ATM cell constructor that includes anillustrative embodiment of the invention;

FIG. 2 is a functional flow diagram of operations of a system sequencerof the ATM cell constructor of FIG. 1;

FIG. 3 is a functional flow diagram of operations of anAAL1-request-sync function of an AAL1-request component of the ATM cellconstructor of FIG. 1;

FIG. 4 is a functional flow diagram of operations of a time slotinterchange (TSI) of the ATM cell constructor of FIG. 1;

FIG. 5 is a functional flow diagram of operations of a digital signalprocessor (DSP) of the ATM cell constructor of FIG. 1;

FIGS. 6-7 are a functional flow diagram of operations of the AA1-requestcomponent of the ATM cell constructor of FIG. 1;

FIG. 8 is a functional flow diagram of operations of anAAL1-request-wideband-sync function of the AAL1-request component of theATM cell constructor,of FIG. 1;

FIG. 9 is a block diagram of an ATM cell;

FIG. 10 is a block diagram of an ATM cell deconstructor that includes anillustrative embodiment of the invention;

FIG. 11. is a functional flow diagram of operations of anAAL1-indication-sync function of an AA1-indication component of the ATMcell deconstructor of FIG. 10;

FIGS. 12-13 are a functional flow diagram of operations of theAAL1-indication component of the ATM cell deconstructor of FIG. 10;

FIG. 14 is a functional flow diagram of operations of a talk time slotinterchange (TSI) component of the ATM cell deconstructor of FIG. 10;and

FIG. 15 is a flow-diagram summary of the functionality of the ATM cellconstructor and deconstructor of FIGS. 1 and 10 that is relevant to theinvention.

DETAILED DESCRIPTION

FIG. 1 shows an ATM cell constructor 100, also known as an ATM cellassembler, such as may be used in an interface port circuit of a PBX orin any other ATM interface apparatus to construct ATM cells from CBRtraffic, such as voice and/or video traffic. Cell constructor 100 andeach of its components may be individually implemented either inhardware or in software/firmware. In the latter case, the software orfirmware may be stored in any desired memory device readable by acomputer—for example, a read-only memory (ROM) device readable by aninterface port circuit processor. Multiple streams (also referred toherein as channels, calls, or communications) of CBR traffic arereceived by ATM cell constructor 100 over a communications medium 102,and follow a data path 150 through ATM cell constructor 100 wheresuccessive segments of the traffic streams are formed into packets (ATMcells). If the switching system employing ATM cell constructor 100 isthe Definity® PBX of Lucent Technologies Inc., medium 102 is atime-division multiplexed (TDM) bus that carries up to 242 individualstreams of traffic in 242 individual time slots of repeating frames.Each frame carries one (narrowband) or more (wideband) time slots ofeach channel's traffic stream. Each time slot carries one byte (octet)of traffic.

A TDM listen interrupt service routine (ISR) 104 captures traffic fromdesignated time listen slots of medium 102 and feeds them serially intoa TDM listen queue 106. A listen time-slot interchanger (TSI) 108retrieves time slots of traffic from TDM listen queue 106 and performsany necessary time-slot interchange function thereon. Listen TSI 108provides support for wideband channels that comprise multiple timeslots; it ensures that those time slots are processed in their properorder. Listen TSI 108 then feeds the reordered time slots of trafficinto one or more digital signal processors (DSPs) 110. A single DSP 110may be time-shared by the plurality of channels, or a separate DSP 110may be dedicated to serving each channel. DSPs 110 perform designatedprocessing for the traffic of each channel, e.g., conferencing, echocancellation, gain adjustment, compression, etc. The processed trafficof each channel is output by DSPs 110 into a separate instance ofAAL1-request processor 112, each dedicated to serving a differentchannel. Each instance of AAL1-request processor 112 constructs ATM cellpayloads from the corresponding channel's received traffic. Whenever itcompletes construction of a single cell's payload, an instance ofAAL1-request processor 112 sends that payload to a correspondinginstance of ATM-request processor 114. There is one instance ofATM-request processor 114 per channel. An instance of ATM-requestprocessor 114 attaches an ATM cell header to the payload to complete theconstruction of an ATM cell and feeds the ATM cell into an ATM queue116. ATM queue 116 is fed by all instances of ATM-request processor 114.An ATM physical layer processor 118 sequentially retrieves cells fromATM queue 116 and transmits them on an ATM communications medium 120toward their destinations.

It takes on the order of a TDM bus frame-interval to process anindividual time slot of traffic through data path 150; of course, up toa frame's worth of time slots may be processed in parallel. A TDM busframe-interval is therefore taken as a cell construction period. It is apredetermined time interval during which each instance of ATM-requestprocessor 114 can mature an ATM cell for transmission. It can take up to47 frames to construct a cell, however.

A control structure 160 controls the operation of the components of datapath 150. Cell constructor 100 receives control information over acontrol medium 122. If the switching system employing cell constructor100 is the above mentioned Definity PBX, control medium 122 isillustratively either a control channel defined by the first 5 timeslots of frames of the TDM bus of the PBX or a packet bus of the PBX.The control information is received in cell constructor 100 by a commandfunction 124. This is a management function which tells controllers126-132 of individual components of data path 150 what their componentshould be doing and when. For example, it tells TSI controller 132 whenlisten TSI 108.should begin to support a new time slot and whichinstance of AAL1-request 112 that time slot should be associated with,it tells controller 126 what VCI/VPI an instance of ATM-request 114should use for a particular channel, it tells controller 128 when toinitialize an instance of AAL1-request 112 for a new channel, and ittells DSP 110 what processing to perform for which channel. Controllers126-132 then exert the corresponding necessary control over theirassociated components in data path 150.

To keep cell constructor 100 properly synchronized with the operation ofcommunications medium 102 in the instance where medium 102 is a TDM bus,a start-of-frame signal is supplied to cell constructor 100 via a signalline 134. Line 134 is monitored by a frame sync interrupt serviceroutine (ISR) 136, which issues an interrupt each time that it detectsthe start-of-frame signal. The interrupt is received by a systemsequencer 138, which is a state machine that causes the components ofdata path 150 to step through their functions during each frame period.

The structure of an ATM cell 1100 assembled by ATM cell constructor 100is shown in FIG. 9. It comprises a conventional 5-octet ATM layer header1102 and a conventional 48-octet payload 1104. The first octet ofpayload 1104 in every ATM cell 1100 is an AAL1 layer header 1106, andthe second octet of payload 1104 in every eighth ATM cell 1100 is aP-format pointer 1108, also as is conventional. AAL1 header 1106conventionally comprises a one-bit convergence sublayer indication C1110, a three-bit call sequence number SEQ 1112, a three-bit cyclicredundancy code over the sequence number CRC 1114, and a one-bit parityindication P 1116. C 1110 is used to provide clock synchronizationbetween the transmitting and receiving equipment. It is set to a “one”on even sequence-count values to indicate P-format payload. SEQ 112 isused by the receiving equipment to detect lost or misinserted cells. P1116 maintains even parity over AAL1 header 1106. P-format pointer 1108comprises a seven-bit SDT (structured data transfer) offset 1120 and aone-bit even-parity indication O 1108. SDT offset is a pointer intopayload 1104 that identifies boundary between blocks (the start of ablock) of data in payload 1104. O 1118 maintains even parity over SDToffset 1120.

High-level functionality of system sequencer 138 is shown in FIG. 2.System sequencer 138 awaits receipt of a frame sync interrupt from framesync ISR 136, at step 200. Upon receipt of the interrupt, systemsequencer 138 starts (e.g., invokes execution of) AAL1-request-sync (aglobal function of AAL1-request 112), at step 202, of TSI 108, at step204. System sequencer 138 then starts command function 124, at step 208.Following step 208 or 210, system sequencer 138 clears the frame syncinterrupt, at step 212, and returns to step 200 to await receipt of thenext frame sync interrupt.

The functionality of the AAL1-request-sync function of AAL1 request 112is shown in FIG. 3. Upon its invocation, at step 300. The functionclears an “SDT OFFSET” global variable 654, at step 303, and thenreturns to the point of its invocation, at step 304.

The high-level functionality of TSI 108 is shown in FIG. 4. Upon itsinvocation, at step 400, TSI 108 retrieves a pointer to its own controldata structure, at step 402, and then uses the pointer to retrieve atime slot identifier plus upper-layer control information for that timeslot from the control data structure, at step 404. The upper-layercontrol information includes information on what processing DSP 110 mustperform on this time slot, and an identifier of the instance ofAAL1-request 112 that this time slot is associated with. TSI 108 thenuses the time slot ID to retrieve the corresponding time slot of trafficfrom TDM queue 106, at step 406, if necessary invokes the time slot'scorresponding instance of DSP 110, at step 408, and passes the trafficand the upperlayer control information to DSP 110, at step 410. TSI 108then increments the pointer to its own control data structure, at step412, and checks whether the pointer points past the last control datastructure entry, at step 414. If not, it means that TSI 108 has not yetprocessed an entire TDM frame of time slots, and so TSI 108 returns tostep 404. If the pointer does point past the end of the control datastructure, it means that TSI 108 has finished processing a whole TDMframe, and so TSI 108 merely resets and stores the pointer, at step 416,and returns to the point of its invocation, at step 418.

The high-level functionality of each instance of DSP 110 is shown inFIG. 5. Upon its invocation, at step 500, the instance of DSP 110receives a time slot of traffic and upper layer control information fromTSI 108, at step 502. DSP 110 then performs the processing specified bythe received control information on the received traffic, at step 504.DSP then invokes the instance of AAL1-request 112 that is specified bythe received control information, at step 506, and passes it the controlinformation and the processed traffic, at step 508. DSP 110 then returnsto the point of its invocation, at step 510.

The high-level functionality of each instance of AAL.1request 112 isshown in FIG. 6. Upon its invocation, at step 600, the invoked instanceof AAL1-request 112 receives an octet of processed traffic andaccompanying control information from DSP 110, at step 602. The invokedinstance of AAL1-request 112 then assembles the received traffic into anATM cell payload, at steps 604-636. If the invoked instance ofAAL1-request 112 presently has a partly-formed cell payload, it adds thereceived traffic to that payload. If the invoked instance of AAL1request 112 presently does not have a partly-formed cell payload, itstarts assembling a new cell payload by creating an AAL1 layer headerbyte 1106 (as well as a P-format pointer 1108 every eighth cell) andattaching the received traffic thereto. AAL1-request 112 checks whetheroctet # 650, an internal variable, is equal to zero, at step 604. If so,there is no partly-formed cell payload. AAL1-request 112 thereforeincrements a sequence # 652, another internal variable, and ANDs it with7 to perform a modulo-8 operation thereon, at step 606, and then checksif the value of sequence # 652 is equal to 0, at step 608. If so,AAL1-request 112 proceeds to form ML1 layer header 1106 and P-formatpointer 1108 for a new ATM cell payload 1104. It sets C bit 1110 inoctet #0 of payload 1104, at step 610; computes and stores CRC 1114 andP 1116 in octet #0, at step 612; sets SEQ 1112 equal to the value ofsequence # 652 in octet #0, at step 614; stores the value of SDT offset654 in SDT offset 1120 of octet #1 of payload 1104, at step 616;computes and stores 0 1118 in octet #1, at step 618; and sets the valueof octet # 650 to two, at step 620. SDT offset 654, it will beremembered, is a global variable, shared by all instances ofAAL1-request 112 and reset at the occurrence of each frame sync signalon TDM bus 102 (see FIG. 3, step 303). Consequently, SDT offset 1120 isalso synchronized with frame sync signals on TDM bus 102, and demarkates the boundaries of TDM frames in the ATM traffic stream.Returning to step 608, if the value of sequence # 652 is odd,AAL1-request 112 proceeds to form AAL1 layer header 1106 only. Itcomputes and stores CRC 1114 and P 1116 in octet #0 of payload 1104, atstep 622, sets SEQ 1112 equal to the value of sequence # 652 in octet#0, at step 624, and sets the value of octet # 650 to one, at step 626.

Having formed the initial one or two bytes of payload 1104 or havingfound at step 604 that there already is a partly-formed cell payload.1104, AAL1-request 112 stores the traffic that it had received at step602 in the octet of payload 1104 pointed to by the value octet # 650, atstep 628. AAL1-request 112 then decrements the value of SDT offset 654,at step 630, and checks if its value is less than zero, at step 632 ofFIG. 7. If so, AAL1-request 112 adds the value of a global variablechannels 656 to SDT offset 654, at step 634. The value of channels 656indicates the number of virtual channels that ATM cell constructor 100is handling. Following step 634, or if it is determined at step 632 thatthe value of SDT offset 654 is not less than zero, AAL1-request 112increments the value of octet # 650, at step 636, and then checks if itis 48, at step 638. If not, assembly of a cell payload 1104 is not yetcompleted, and so AAL1-request 112 merely returns, at step 649; if so,assembly of a cell payload 1104 is completed, and so AAL1 request 112sets the value of octet # 650 to zero, at step 640, invokes acorresponding instance of ATM-request 114, at step 642, and passes to itthe completed ATM cell payload, at step 644. AAL1-request 112 thenreturns, at step 649.

The high-level functionality of the AAL1-request-wideband-sync functionof AAL1-request 112 is shown in FIG. 8. Upon its invocation, at step1000, the function sets a loop-counter variable equal to the totalnumber of wideband channels presently being processed by all instancesof AAL1-request 112, at step 1002, and checks if the value of the loopcounter is zero, at step 1003. If the loop-counter value is zero, itmeans that the function has analyzed all instances of AAL1-request 112that are presently processing wideband channels, and so the functionreturns to the point of its invocation, at step 1018. If theloop-counter value is not zero, the function proceeds to step 1004 toanalyze the next instance of AAL1-request 112 that is processing awideband channel. At step 1004, the function retrieves a pointer to thefirst instance of AAL1-request 112 that is presently processing awideband channel, and from that instance obtains an assembly pointerthat points to the next ATM cell octet which the instance ofAAL1-request 112 will assemble in the cell payload during the next frameinterval, at step 1006. The function then increments the assemblypointer by the number of narrowband channels (time slots per frame) thatconstitute the wideband channel (it is assumed here that all widebandchannels have the same known size), at step 1008, decrements the loopcounter, at step 1014, and returns to step 1003 to determine if it hasanalyzed all instances of AAL1-request 112 that are presently processingwideband channels.

FIG. 10 shows an ATM cell deconstructor 2100, also known as an ATM cellreassembler, such as may be and in an interface port circuit of a PBX orin any other ATM interface apparatus to deconstruct ATM cells into CBRtraffic streams. Like constructor 100, deconstructor 2100 may beimplemented either in hardware or in software/firmware. Deconstructor2100 and its parts are functionally mirror images of constructor 100 andits parts. ATM cells are sequentially received on ATM communicationsmedium 120 by an ATM physical layer 2118 and follow a data path 2150through ATM cell deconstructor 2100 where the cells are broken down intosuccessive segments of CBR traffic streams, and the traffic streams aretransmitted on communications medium 102. ATM physical layer 2118 feedsreceived ATM cells into an ATM queue 2116, which supplies them toATM-indication processor 2114. There is one instance of ATM-indicationprocessor 2114 per channel. An instance of ATM-indication processor 2114detaches the ATM cell header from ATM cells carrying its channel'straffic, and sends the ATM cell payload to a corresponding instance ofAAL1-indication processor 2112, each one of which instances is dedicatedto serving a different channel. Each instance of AAL1-indicationprocessor 2112 extracts the corresponding channel's octets of trafficfrom the received payloads and outputs the octets to one or more DSPs2110 for processing. A single DSP 2110 may be time-shared by theplurality of channels, or a separate DSP 2110 may be dedicated toserving each channel. DSPs 2110 perform designated processing for thetraffic of each channel, and output the processed traffic into a talkTSI 2108. Talk TSI 2108 performs any necessary time-slot interchangefunction thereon to align the traffic octets of different channels withthe sequence of their corresponding time slots on medium 102. Talk TSI2108 also provides support for wideband channels that comprise multipletime slots; it ensures that those time slots are output in their properorder. Talk TSI 2108 then feeds the ordered time slots of traffic into aTDM talk queue 2106, from where they are transmitted onto designatedtime slots of frames on medium 102 by TDM talk ISR 2104.

A control structure 2160 controls the operation of the components ofdata path 2150. Cell deconstructor 2100 receives control informationover a control medium 2122, which illustratively duplicates controlmedium 122 of cell constructor 100. The control information is receivedin cell deconstructor 2100 by a command function 2124. This is amanagement function which tells controllers 2126-2132 of individualcomponents of data path 2150 what their component should be doing andwhen. For example, it tells TSI controller 2132 when talk TSI 2108should begin to support a new time slot and which instance ofAAL1-indication 2112 that time slot should be associated with, it tellscontroller 2126 what VCINPI an instance of ATM-indication 2114 shoulduse for a particular channel, it tells controller 2128 when toinitialize an instance of AAL1-indication 2112 for a new channel, and ittells DSP 2110 what processing to perform for which channel. Controllers2126-2132 then alert the corresponding necessary control over theirassociated components in data path 2150.

To keep cell deconstructor 2100 properly synchronized with the operationof communications medium 102 in the instance where medium 102 is a TDMbus, a start-of-frame signal is supplied to cell deconstructor 2100 in asignal line 134. Line 134 is monitored by a frame sync ISR routine 2136,which issues an interrupt each time that it detects the start-of framesignal. The interrupt is received by a system sequencer 2138, which is astate machine that causes the components of data path 2150 to stepthrough their functions during each frame period. Upon receipt of theinterrupt, system sequencer 2138 also starts (e.g., invokes executionof) AAL1-indication-sync function (a global function of AAL1-indication2112). The functionality of the AAL1-indication-sync function is shownin FIG. 11. Upon its invocation, at step 1400, the function clears an“STD offset” 1654, a global variable of ATM cell deconstructor 2100, atstep 1404, and then returns to the point of its invocation, at step1406.

The high-level functionality of each instance of AAL1-indication 2112 isshown in FIGS. 12-13-16. Upon its invocation, at step 1500, the invokedinstance of AAL1 indication 2112 proceeds to disassemble an octet froman ATM cell payload 1104. AAL1-indication 2112 first checks whether thevalue of octet # 1650, a global variable of ATM cell deconstructor 2100,is zero, at step 1502. If not, AAL1-indication 2112 is in the midst ofdisassembling an ATM cell payload 1104, and so it proceeds to FIG. 13 todisassemble another octet from the payload. If the value of octet # 1650is zero, AAL10 indication 2112 has completed disassembly of an ATM cellpayload 1104 and is ready to start disassembly of a next ATM cellpayload 1104. It therefore proceeds to receive an ATM cell payload 1104from a corresponding instance of ATM-indication 2114, at step 1504.AAL1-indication 2112 then retrieves SEQ 1112 from octet #0 of thatpayload, at step 1506, and checks if its value equals the value ofsequence # 1652, at step 1508. If the two values are not equal, it meansthat an ATM cell has been lost in transmission, and so AAL1-indication2112 increments a lost cell error counter 1658 to give an indication ofthe loss, at step 1510. Following step 1510, or if the value of SEQ 1112is found to equal the value of sequence # 1652 at step 1508,AAL1-indication 2112 increments sequence # 1652 and ANDs the incrementedvalue with 7 to effect a modulo-8 operation thereon, at step 1512.AAL1-indication 2112 then checks if C bit 1110 of octet #0 of the ATMcell payload 1104 that is being disassembled is set and SEQ 1112 iseven, at step 1514. If so, set it means that octet #1 of that payload isa P-format pointer 1108. AAL1-indication 2112 therefore retrieves SDToffset 1120 from octet #1, at step 1516, sets the value of SDT offset1654 to the retrieved value of SDT offset 1120 to ensure that the valueof SDT offset 1654 is in sync with the value of SDT offset 1120, at step1518, and then sets the value of octet # 1650 to two, at step 1520.Returning to step 1514, if the condition is not met, the second octet ofthat payload is an ordinary payload octet. AAL1-indication 2112therefore sets the value of octet # 1650 to one, at step 1522.

Following step 1520 or 1522, or if it was determined at step 1502 thatthe value of octet # 1650 is not zero, AAL1-indication 2112 proceeds toretrieve from the ATM payload 1104 being disassembled the octet pointedto by the value of octet # 1650, at step 1530 of FIG. 13.AAL1-indication 2112 then decrements SDT offset 1654, at step 1532, andchecks if the decremented value is less than zero, at step 1534. If so,it means that a block of information (a frame of channels) has beendisassembled and disassembly of a new block is beginning, or that a newframe is beginning on TDM bus 102. AAL1-indication 2112 therefore addsthe value of channels 656 to SDT offset 1654 to reinitialize SDT offset1654, at step 1536, and also sets a “first channel” flag that willaccompany the just-retrieved octet to subsequent elements 2104-2110 ofdata path 2150, at step 1538. Following step 1538, or if it wasdetermined at step 1534 that the value of SDT offset 1654 is not lessthan zero, AAL1-indication 2112 increments octet # 1650, at step 1540,and checks if the incremented value exceeds 48, at step 1542. If so, itmeans that disassembly of this ATM cell payload 1104 has been completed,so AAL1-indication 2112 resets the value of octet # 1650 to zero, atstep 1544. Following step 1544, or if it was determined at step 1542that the value of octet # 1650 does not exceed 48, AAL1-indication 2112returns to the point of its invocation, at step 1546.

The high-level functionality of talk TSI 2108 is shown in FIG. 14. Uponits invocation, at step 1700, TSI 2108 initializes a TSI pointer 1750.TSI pointer 1750 points into a TSI control memory to a location thatspecifies the received time slot which the TSI should presently beoutputting. TSI 2108 also initializes a loop counter 1752 to the valueof channels 656, at step 1704. Loop counter 1752 is thus initialized tothe number of channels of traffic carried by received frames. TSI 2108then checks is the value of loop counter 1752 is greater than zero, atstep 1706. If not, TSI 2108 has complete time-slot interchanging of aframe, and so it returns to the point of its invocation, at step 1722.But if the value of loop counter 1752 is greater than zero, TSI 2108continues its function by retrieving, from a buffer of DSPs 2110, thetime slot (octet of traffic) pointed to by TSI pointer 1750, at step1708. TSI 2108 checks if the retrieved time slot is accompanied by a“first channel” flag, at step 1710. If so, TSI 2108 resets itself to astart-of-frame-processing by reinitializing TSI pointer 1750 to zero, atstep 1712, and by reinitializing loop counter 1752 to the value ofchannels 656, at step 1714. TSI 2108 thus ensures that its operation issynchronized with the occurrence of frame boundaries in both the streamof incoming traffic and the TDM bus 102.

After step 1714, or if it is determined at step 1710 that the retrievedtime slot is not accompanied by a “first channel” flag, TSI 2108 outputsthe retrieved time slot to TDM talk queue 2106, at step 1716. TSI 2108then decrements the value of loop counter 1752, at step 1718, incrementsthe value of TSI pointer 1750, at step 1720, and returns to step 1706 todetermine if it is done processing a full frame of traffic.

The high-level operations of ATM cell constructor 100 and deconstructor2100 that are relevant hereto are summarized in FIG. 15. Constructor 100receives a piece of information (traffic) from TDM bus 102, at step1802, and assembles it into an ATM cell, at step 1804. If the piece ofinformation is accompanied by a frame signal on listen bus 102, asdetermined at step 1806, constructor 100 includes a SDT offset pointer1120 to that piece of information in the ATM cell that is beingassembled, at step 1810. After step 1806 or 1810, constructor 100 checksif it is done assembling a cell; if not, it returns to step 1802, and ifso, it transmits the ATM cell on ATM link 120, at step 1812.

The transmitted ATM cells are received by deconstructor 2100, at step1814, which disassembles the information (traffic) therefrompiece-by-piece, at step 1816. Deconstructor 2100 checks whether thedisassembled information is pointed to by SDT offset pointer 1120, atstep 1818, or accompanied by a frame signal on talk bus 102, at step1819. If so, talk TSI 2108 resets, at step 1820, and starts processing anew TDM frame, at step 1821. Following step 1819 or 1821, talk TSI 2108effectively adds the piece of information to the frame on talk bus 102,that it is processing by outputting it to TDM talk queue 2106 fortransmission on talk TDM bus 102. TSI 2108 then checks whether it hascompleted outputting a TDM frame, at step 1824. If not, operationreturns to step 1816; if so, TSI 2108 starts processing a new TDM frame,at step 1826, and operation returns to step 1816.

Of course, various changes and modifications to the illustrativeembodiment described above may be envisioned. For example the inventionis applicable not only to ATM systems, but also to framerelay,voice-over-IP (internet protocol), and other systems that supportsynchronous constant-bit rate (e.g., voice, video, data) connections.Such changes and modifications can be made without departing from thespirit and the scope of the invention and without diminishing itsattendant advantages. It is therefore intended that such changes andmodifications be covered by the following claims.

What is claimed is:
 1. A method of synchronizing synchronous informationreceived by a receiver over an asynchronous communications link with asynchronous communications medium at the receiver, comprising:asynchronously receiving at the receiver a stream of informationincluding the synchronous information and an indication of where in thestream of information occurred a boundary between blocks of information;transmitting the stream of information from the receiver on the medium;detecting at the receiver where boundaries between blocks forinformation occur on the medium; and in response to receipt of theindication; synchronizing transmissions from the receiver on the mediumof information at the boundary indicated by the indication in the streamof information with occurrence of a boundary between blocks on themedium.
 2. The method of claim 1 wherein: the indication indicates wherein the synchronous information occurred the boundary between blocks ofthe synchronous information.
 3. The method of claim 1 wherein:asynchronously receiving comprises repeatedly receiving indications ofoccurrences of boundaries between sequential blocks of the stream ofinformation; and synchronizing comprises in response to receipt of eachindividual indication, synchronizing the transmission from the receiveron the medium of information at the boundary indicated by the individualindication in the stream of information with occurrence of a boundarybetween blocks on the medium.
 4. The method of claim 3 wherein: eachboundary between blocks of information and between blocks forinformation comprises a boundary between adjacenttime-division-multiplex (TDM) frames of information.
 5. The method ofclaim 4 wherein: each indication points to information carried by afirst time slot of a TDM frame; transmitting from the receiver comprisestime-slot interchanging the information to form new TDM frames ofinformation, and transmitting the new TDM frames from the receiver onthe medium in TDM frames of the medium; and synchronizing comprises inresponse to receipt of each indication, resetting the time-slotinterchanging to form a new TDM frame starting with the informationpointed to by the indication.
 6. The method of claim 1 wherein: themedium is a time-division-multiplexed (TDM) medium; the indication is anindication of occurrence of a TDM framing signal; and detectingcomprises detecting TDM frame signals on the medium.
 7. The method ofclaim 1 wherein: asynchronously receiving comprises receiving the streamof information in packets.
 8. The method of claim 7 wherein: the packetscomprise asynchronous transfer mode (ATM) cells.
 9. The method of claim8 wherein: the indication comprises a structured data transfer (SDT)offset pointer of an AAL payload of an ATM cell.
 10. The method of claim9 wherein: the medium is a time-division-multiplexed (TDM) medium;detecting comprises detecting TDM frame signals on the medium; and theSDT pointer points to information carried by a first time slot of a TDMframe of information.
 11. A method of synchronizing a first synchronouscommunications medium at a receiver of an asynchronous communicationslink with a second synchronous communications medium at a transmitter ofthe asynchronous communications link, comprising: detecting at thetransmitter occurrence of a boundary between blocks of information in astream of information being received from the second medium at thetransmitter; transmitting the information asynchronously from thetransmitter to the receiver with a first indication of where in thestream of information the boundary occurred; receiving the informationand the first indication at the receiver; transmitting a stream of theinformation from the receiver on the first medium; detecting at thereceiver where boundaries between blocks for information occur on thefirst medium; in response to receipt of the first indication,synchronizing transmission from the receiver on the first medium of theinformation at the boundary indicated by the first indication withoccurrence of a boundary between blocks on the first medium.
 12. Themethod of claim 11 wherein: detecting at the transmitter comprisesrepeatedly detecting occurrence of boundaries between sequential blocksof the information; transmitting from the transmitter comprises inresponse to each individual detected boundary, transmitting the firstindication of where in the information the individual boundary occurred;receiving at the receiver comprises repeatedly receiving the firstindication at the receiver; and synchronizing comprises in response toreceipt of each individual first indication, synchronizing thetransmission from the receiver on the first medium of the information atthe individual boundary indicted by the individual first indication withoccurrence of a boundary between blocks on the first medium.
 13. Themethod of claim 12 wherein: each boundary between blocks of informationand between blocks for information comprises a boundary between adjacenttimedivision multiplex (TDM) frames.
 14. The method of claim 13 wherein:each first indication points to information carried by a first time slotof a TDM frame; transmitting from the receiver comprises time-slotinterchanging the information to form TDM frames of information, andtransmitting the TDM frames of information from the receiver on thefirst medium; and synchronizing comprises in response to receipt of thefirst indication, resetting the time-slot interchanging to form a TDMframe starting with the information pointed to by the first indication,and transmitting each formed TDM frame in a TDM frame of the firstmedium.
 15. The method of claim 11 wherein: the first and the secondmedia are time-division-multiplexed (TDM) media; and each detectingcomprises detecting TDM frame signals.
 16. The method of claim 11wherein: transmitting the information asynchronously comprisestransmitting the information in packets.
 17. The method of claim 16wherein: the packets comprise asynchronous transfer mode (ATM) cells.18. The method of claim 17 wherein: the first indication comprises astructured data transfer (SDT) offset pointer of an AAL payload of anATM cell.
 19. The method of claim 18 wherein: the first and the secondmedia are time-division-multiplexed (TDM) media; each detectingcomprises detecting TDM frame signals; and the SDT offset pointer pointsto information carried by a first time slot of a TDM frame.
 20. Acommunications apparatus for performing the method of claim 1 or 2 or 3or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13 or 14 or 15 or 16or 17 or 18 or
 19. 21. A computer-readable medium containing softwarewhich, when executed in a computer, causes the computer to perform themethod of claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10.